President Donald Trump questioned the integrity of special counsel Robert Mueller's investigation in a weekend tweetstorm and doubled down on his claims that there was no collusion during the 2016 election.

"The Mueller probe should never have been started in that there was no collusion and there was no crime. It was based on fraudulent activities and a Fake Dossier paid for by Crooked Hillary and the DNC, and improperly used in FISA COURT for surveillance of my campaign. WITCH HUNT!" Trump tweeted late Saturday.

The Mueller probe should never have been started in that there was no collusion and there was no crime. It was based on fraudulent activities and a Fake Dossier paid for by Crooked Hillary and the DNC, and improperly used in FISA COURT for surveillance of my campaign. WITCH HUNT!

He followed up Sunday morning by claiming that Mueller's team was comprised of "13 hardened Democrats, some big Crooked Hillary supporters, and zero Republicans."

"Does anyone think this is fair?" Trump questioned. "And yet, there is NO COLLUSION!"

Why does the Mueller team have 13 hardened Democrats, some big Crooked Hillary supporters, and Zero Republicans? Another Dem recently added...does anyone think this is fair? And yet, there is NO COLLUSION!

Some of Mueller's investigators have made contributions to Clinton and other Democratic political candidates, but those contributions were lawful, the Associated Press reported. Justice Department policy and federal service law both prohibit discrimination in the hiring of career positions on the basis of political party affiliation, the AP added.

In response to what some perceived as a threat to fire Mueller, several Senate Republicans voiced support for Mueller's investigation and urged Trump not to fire him during the Sunday morning talk circuit.

"I just hope it doesn't go there because it can't. We can't, in Congress, accept that. I would expect to see considerable pushback in the next couple of days urging the president not to go there. He can't go there," Sen. Jeff Flake, R-Ariz., said during an appearance on CNN.

"I don't see the president firing him. I think the White House has said 10 times, maybe more, that they are not going to fire Robert Mueller. They want him to be able to finish the investigation," Sen. James Lankford, R-Okla., told ABC on "This Week."